Method and apparatus for checking sensors

ABSTRACT

Methods and apparatus are disclosed for testing differential pressure sensors. The method includes applying a pressure of predetermined value to the reference side of the differntial pressure sensors, measuring the output values from the measurement sides of the differential pressure sensors, and comparing the measured output values. The disclosed apparatus includes a pump for simultaneously applying a pressure of a predetermined value to the reference side of a number of differential pressure sensors.

FIELD OF THE INVENTION

The present invention relates to a method and apparatus for testingsensors of the type for measuring a specific quantity. Moreparticularly, the present invention relates to a method for testingdifferential pressure sensors in an apparatus for the extracorporealtreatment of blood, such as a dialysis monitor.

BACKGROUND OF THE INVENTION

In apparatus for the extracorporeal treatment of blood, there areusually arranged several identical sensors for measuring a specificquantity. This is intended to achieve improved control and monitoring,as well as increased safety. For example, a dialysis monitor comprisesseveral sensors for measuring the pressure in fluid conduits utilizedduring dialysis.

On the one hand, these fluid conduits transport blood from the patientto the blood side of the dialyser and from there back to the patientand, on the other hand, they transport dialysis fluid from the dialysisfluid source to the dialysate side of the dialyser, and from there to anoutlet. The blood-carrying conduits are generally exchanged after eachtreatment, while the conduits transporting the dialysis fluid arepermanently connected to the dialysis monitor and are disinfected andrinsed after each treatment.

The monitoring of pressure during dialysis, both in the blood-carryingconduits and in the conduits carrying the dialysis fluid, is importantfor several reasons. One important reason is, for example, surveillanceof the transmembrane pressure at the membrane present in the dialyserthat separates dialysis fluid from blood, and which is semi-permeable.This transmembrane pressure is the pressure difference between thepressure on the blood side and the pressure on the dialysate side, anddetermines the direction and quantity of fluid transport through themembrane. If the transmembrane pressure is positive, then the fluid willbe drawn out of the blood. This process is also called ultra-filtration.With a negative transmembrane pressure no excess fluid will be drawnfrom the patient. The removal of excess fluid is, however, one of theprincipal functions of dialysis, so that the importance of monitoringthe pressure in the dialysis fluid conduits and blood conduits isevident from this alone.

A further important reason is monitoring of the arterial and venalpressure in the blood-carrying conduits during treatment. If, forexample, the arterial pressure should be below a specific negativepressure, then the arterial needle could be blocked or lodged in thefistula wall, or the conduit could be folded over. In order to avoiddamage to the fistula, the machine will automatically be stopped, and analarm triggered. If the arterial pressure exceeds a predeterminednegative pressure, then the arterial needle could have come loose and besucking in air. In this case also an alarm will automatically betriggered, and the machine stopped.

On the other hand, if the venal pressure should fall below apredetermined value, then the blood feedback to the patient could be notsealed or interrupted. If the venal pressure exceeds a specificpredetermined value, then the venal needle could be blocked, or thefeedback tube bent. In this case the machine will likewise be stopped,and an alarm automatically triggered.

However, in order to reliably monitor the pressure, operation of thesensors must be trouble-free. These are thus checked prior to operationof the dialysis monitor during a check phase, in which a functionalcheck of the entire dialysis monitor takes place.

Thus, for checking the sensors that measure the pressure in the dialysisfluid conduits (hereinafter referred to as dialysate pressure sensors),at least two of which are provided, a static pressure is generated inthese conduits. For the present case, static pressure means that thedialysis fluid does not flow through the conduits, but rather sits inthe conduits, and that a pressure is applied to the stationary dialysisfluid. In this way, the effects on the pressure measurement of thevalves, throttles, pumps, etc. that are arranged in the tubes, areexcluded. The values supplied by the individual dialysate pressuresensors are compared with one another; the individual dialysate pressuresensors must all supply the same value, as they are subjected to thesame pressure.

If the individual dialysate pressure sensors supply different values,then a sensor malfunction has occurred, and the control or monitoringunit of the dialysis monitor generates a corresponding alarm.

The pressure sensors generally used in this case are so-calleddifferential pressure sensors that utilize atmospheric pressure as areference value. These sensors are checked by applying two differentstatic pressures in the described manner, one generally being positivewith respect to atmospheric pressure and one being negative with respectto atmospheric pressure. On the one hand, the values supplied by thesensors are compared with one another in order to determine a possiblesensor malfunction. On the other hand, the values supplied by eachindividual sensor at the different pressures are also used to determinethe constant allocated to each individual sensor. This constant isnecessary for precise determination of the pressure, and can bedetermined only with the aid of at least two values supplied atdifferent pressures.

The sensors that measure the pressure in the blood conduits, of which atleast two are provided, (hereinafter referred to as blood pressuresensors) are usually also differential pressure sensors. In contrast tothe dialysate pressure sensors that are permanently connected with thedialysis fluid conduit, the blood pressure sensors are not permanentlyconnected to the blood conduits. As mentioned above, new blood conduitsare used for each treatment, so that the blood pressure sensors must beconnected to the blood conduits by the operating personnel before eachtreatment. During the check phase before the treatment, the bloodpressure sensors are generally not connected to the blood conduits, thusno pressure or different pressures may be generated in the bloodconduits and applied to the blood pressure sensors for testing same. Itis, therefore, only possible to examine whether the blood pressuresensors each supply the same reference value corresponding toatmospheric pressure. However, this provides no information as towhether the blood pressure sensors will supply the same values at thesame applied pressure above or below atmospheric pressure.

It is possible to test the blood pressure sensors during the check ortest phase by connecting the blood pressure sensors with a separateconduit and to generate one predetermined pressure, or differentpredetermined pressures, in this conduit. However, this requires, on theone hand a supplementary material complexity, as the dialysis monitormust be correspondingly adapted to enable this additional test option.Furthermore, additional, specially constructed conduits are necessary,so that the material complexity and the cost as a whole increases. Onthe other hand, the operating complexity for the operating personnel ismarkedly increased as, in the test phase, the operating personnel mustfirst attach the special test tube to the dialysis monitor and connectit to the blood pressure sensors. After completing the test, theoperating personnel must then remove the special test tube before theblood conduits and the dialyser can be connected to the dialysismonitor, and the blood pressure sensors can be connected to the bloodconduits.

In view of this background, it is an object of the present invention toprovide a method and apparatus for testing identical sensors formeasuring a specific quantity, in particular for checking differentialpressure sensors with a reference side and a measurement side, withwhich these disadvantages can be alleviated.

In particular, testing of the blood pressure sensors of dialysismonitors without additional manipulation by the operating personnel, andwithout the attachment of separate tubes to the dialysis monitor thatneed removing after testing, should be made possible.

SUMMARY OF THE INVENTION

In accordance with the present invention, these and other objects havenow been realized by the invention of a method for testing a pluralityof differential pressure sensors, each of the plurality of differentialpressure sensors including a reference side and a measurement side, themethod comprising applying a pressure having a predetermined value tothe reference side of the plurality of differential pressure sensors,measuring the output values from the measurement side for each of theplurality of differential pressure sensors, and comparing the pluralityof measured output values. Preferably, the predetermined value comprisesa positive pressure value. In another embodiment, the predeterminedvalue comprises a negative pressure value.

In accordance with one embodiment of the method of the presentinvention, the predetermined value comprises a first predeterminedvalue, and the method includes applying a pressure having a secondpredetermined value to the reference side of the plurality ofdifferential pressure sensors. Preferably, one of the first and secondpredetermined values comprises a positive pressure value and the otherof the first and second predetermined values comprises a negativepressure value.

In accordance with one embodiment of the method of the presentinvention, the method includes applying the pressure by means of a pump.

In accordance with another embodiment of the method of the presentinvention, applying of the pressure is carried out in an apparatus forthe extracorporeal treatment of blood. Preferably, applying the pressureis carried out in a dialysis monitor.

In accordance with the present invention, these objects have also beenrealized by the invention of apparatus for testing a plurality ofdifferential pressure sensors, each of the plurality of differentialpressure sensors including a reference side and a measurement side, theapparatus comprising pressure means for simultaneously applying apressure having a predetermined value to the reference side of theplurality of differential pressure sensors. Preferably, the pressuremeans comprises pressure generating means. In a preferred embodiment,the apparatus includes connection means for connecting the pressuregenerating means to the plurality of differential pressure sensors.Preferably, the connecting means comprises tube means, preferablycomprising pipe means.

In accordance with one embodiment of the apparatus of the presentinvention, the pressure means comprises a pump. In a preferredembodiment, the pump is selected from the group consisting of a plungerpump and a peristaltic pump.

In accordance with one embodiment of the apparatus of the presentinvention, the connection means comprises permanent connection means.

In accordance with another embodiment of the apparatus of the presentinvention, the connection means connects the pressure generating meansto the plurality of differential pressure sensors solely for testing thedifferential pressure sensors. Preferably, the connection means includesan equalization opening dimensioned such that the pressure means canapply the pressure having the predetermined value to the reference sidesof the plurality of differential pressure sensors and the surroundingpressure can be applied to the reference sides of the plurality ofdifferential pressure sensors as a reference value.

The objects of the present invention are achieved by a method, wherein apressure of predetermined value is applied simultaneously to thereference side of the sensors, and the output values of the sensors arecompared with one another.

In this manner, a simple and reliable method is provided, with which thefunctional capability of differential pressure sensors can be determinedwith certainty. If a predetermined pressure value is appliedsimultaneously, for example, to the reference side of the blood pressuresensors in a dialysis monitor, then these must supply the same outputvalue, or the same increase or decrease in output value, if they arefunctioning correctly. By comparing the output values of the sensors,the functioning of the sensors can, therefore, easily be verified. Ifthe blood pressure sensors supply the same output value, or the sameincrease or decrease in output value, then no malfunction has occurred.However, if the blood pressure sensors supply different output values,or different variations in output value, then a malfunction has occurredand corresponding measures for removing the error can be applied.

The pressure values applied during this process can be of any desiredmagnitude and should advantageously lie in the working range, ormeasurement range, of the utilized differential pressure sensors. Forexample, the pressure value can be a positive or a negative value.

However, when using two different pressure values, on the one hand, theprecision of the test can be increased, and on the other hand, theconstant associated with each pressure sensor can be determined, asalready described in detail above. The constant associated with eachdifferential pressure sensor is necessary for exact determination of thepressure and can be determined only with the aid of at least twomeasurement values. Thus, according to a preferred embodiment of thepresent invention, two different predetermined pressure values aresuccessively applied to the reference side of the differential pressuresensors.

These different pressure values can be any desired pressure value, butadvantageously a pressure that is positive relative to atmosphericpressure and a pressure that is negative relative to atmosphericpressure are applied successively, wherein either the positive or thenegative pressure can be applied first. This is particularlyadvantageous for blood pressure sensors in a dialysis monitor which mustmeasure both the negative arterial pressure and the positive venalpressure in the blood carrying conduits, so that the working range ofthe sensors is tested.

The pressure applied to the reference side for testing the differentialpressure sensors can be generated by any appropriate means. However, themeans should be selected such that they generate a pressure lying in theworking, or measurement, range of the differential pressure sensors.Advantageously, the pressure is generated by means of a pump, as largepressure ranges can be covered thereby.

With the method according to the present invention, differentialpressure sensors that are arranged in an apparatus for theextracorporeal treatment of blood can advantageously also be tested. Inthese apparatus, and particularly in dialysis monitors, there are interalia generally several differential pressure sensors provided formeasuring the pressure in the blood tubes. With the method according tothe present invention, for example, these blood pressure sensors can beeasily tested, without large additional effort by the operatingpersonnel and without large supplementary material costs.

The objects of the present invention are achieved by means of anapparatus, wherein the apparatus comprises means for simultaneouslyapplying a predetermined pressure to the reference side of the sensors.

In this manner, simple and reliable apparatus is provided, with whichthe functional capability of differential pressure sensors can bedetermined with certainty. If a predetermined pressure is appliedsimultaneously to the reference side of, for example, the blood pressuresensors in a dialysis monitor with the aid of the means, then thesensors should supply the same output value, or increase or decrease inoutput value, when functioning correctly. As described above in detailwith reference to the solution in the form of a method, the function ofthe blood pressure sensors can be easily checked by comparing the outputvalues of the sensors with one another.

The apparatus does not need to include means for generating the pressureitself, although this is provided according to a preferred embodiment.The means for generating the pressure itself can take any desired form,provided that they ensure generation of the required pressure.Advantageously, however, the means for generating the pressure comprisea pump, so that large pressure ranges may be covered. According to aparticularly preferred embodiment, the pump is a plunger pump or aperistaltic pump, which, for example, is widely used in the field ofmedicine.

The means for simultaneously applying the predetermined pressure valueto the reference side of the sensors can take any desired form, providedthat they ensure simultaneous application to the reference side.However, advantageously, the means for simultaneous application comprisemeans for connecting the sensors with means for generating the pressure.In this manner, it is possible to apply the generated pressure directlyto the reference side of the sensors. According to a further preferredembodiment, therefore, means for connecting the sensors with means forgenerating the pressure comprise tubes, while according to anotherpreferred embodiment they comprise pipes.

These pipes or tubes can permanently connect the means for generatingthe pressure with the sensors, which is advantageous when, for example,the apparatus for testing the sensors is to be permanently arranged in adialysis monitor. This offers the advantage that the blood pressuresensors can be checked at any time by the operating personnel, withoutadditional manipulation. This means not only that the blood pressuresensors can be automatically tested in the test phase of the dialysismonitor prior to dialysis treatment but, for example, also during thedialysis treatment itself, or during short interruptions in thetreatment, during which the dialysis monitor is checked and calibrated.Thus, a further preferred embodiment provides that the pipes or tubespermanently connect the means for generating the pressure with thesensors.

However, when manipulation by the operating personnel is taken intoaccount, it is also possible to connect means for generating thepressure with the sensors by means of pipes or tubes only for thepurpose of testing the sensors. This is useful, for example, when oneapparatus for testing sensors is to be used with several devices, suchas dialysis monitors. This flexible utilisation of the apparatus fortesting sensors is, therefore, provided in accordance with anotherpreferred embodiment, in which the means for connecting the sensors withthe means for generating the pressure values only connect the means forgenerating the pressure values with the sensors for the purpose oftesting the sensors.

The atmospheric, or surrounding, pressure that acts on the referenceside of the differential pressure sensors and is utilized as a referencevalue for the differential pressure sensors can be applied to thereference side in any desired manner. However, it is advantageous whenthe atmospheric, or surrounding, pressure also reaches the referenceside through the connecting means, as this provides a defined access tothe reference side. At the same time, the connecting means can comprisean equalisation opening that is dimensioned such that, on the one hand,the predetermined pressure can be applied to the reference side by themeans for generating the pressure, and on the other hand, thesurrounding pressure can be applied to the reference side as a referencevalue. This is advantageous when the means for generating the pressurecloses the connecting means, so that, apart from the equalisationopening, the reference side of the sensors has no direct connection tothe surroundings, which, for example, is the case for a peristaltic orplunger pump. In this case, the equalisation opening provides a definedconnection to the surroundings.

DESCRIPTION OF THE DRAWINGS

In the following detailed description, the present invention will bedescribed in more detail with reference to the enclosed drawings, inwhich:

FIG. 1 is a front, schematic representation of apparatus for testingsensors,

FIG. 2 is a front, perspective view of a sensor used in the apparatusshown in FIG. 2, and

FIG. 3 is a top, elevational, schematic representation of a peristalticpump utilized in the apparatus of the present invention.

DETAILED DESCRIPTION

Referring to the drawings, FIG. 1 schematically shows apparatus fortesting differential pressure sensors, the apparatus being permanentlyarranged in a dialysis monitor. The dialysis monitor is shown with onlypart of its housing. In the housing, or within the housing wall 10,three differential pressure sensors, 12, 14, and 16, are arranged formeasuring the pressure in blood conduits (not shown).

It is to be noted, that the differential pressure sensors, 12, 14, and16, can be arranged in a circuit or on a printed circuit within themonitor housing, or each differential pressure sensor can be arranged onits own printed circuit. In this case (not shown), the measurement sidesof each of the differential pressure sensors would be connected toterminals on the housing wall in a known manner using appropriateconduits. These connecting conduits may comprise blood filters in orderto prevent contamination of the sensors, and thus endangerment of thefollowing patient if the blood conducting tubes leak.

The differential pressure sensors, 12, 14, and 16, each comprise a tubeconnection on their reference side, which lies inside the housing 10.The tube connection 20 of the differential pressure sensor 12 isconnected with an end of a connecting tube 26, as is the tube connection24 of the differential pressure sensor 14, and the tube connection 22 ofthe differential pressure sensor 16. This connecting tube 26 leads to aperistaltic pump 18, and is inserted in the peristaltic pump in such amanner that a suction opening 60 at the other end of the connecting tube26 lies free. The differential pressure sensors, 12, 14, and 16, are,furthermore, connected to a control unit or monitoring unit 40 by meansof electric leads, 42, 44, and 46. The peristaltic pump 18 is alsoconnected to the control, or monitor, unit 40 by means of an electriclead 48.

In order to test the differential pressure sensors, 12, 14, and 16, theperistaltic pump 18 is started under the control of control unit 40. Itthus sucks air through the suction opening 60 into the connecting tube26, as indicated by the arrow 62, and generates an excess pressure inthe connecting tube 26. This excess pressure is further carried to thetube connections, 20, 22, and 24, connected with connecting tube 26. Thetube connections are each connected to the reference side of thedifferential pressure sensors, 12, 14, and 16, respectively, so that thegenerated pressure acts on the reference side of the differentialpressure sensors, 12, 14, and 16. The resulting signal supplied by eachof the differential pressure sensors, 12, 14, and 16, is sent to thecontrol, or monitoring, unit 40 by means of the electrical leads 42, 44,46. The control unit, or monitor 40 compares the values supplied by thesensors, 12, 14, and 16, and signals an error when the supplied signalsare not equal. When functioning correctly, the sensors, 12, 14, and 16,each supply the same signal to the control, or monitoring, unit 40, asthe same pressure acts on the differential pressure sensors, 12, 14, and16, or rather, their reference side.

After terminating the test of the differential pressure sensors, 12, 14,and 16, the peristaltic pump 18 is turned off under control of thecontrol, or monitoring, unit 40. The stationary, or halted, peristalticpump 18 closes the suction opening 60 of the connecting tube 26, so thatpressure equalisation with the surroundings is not possible through thesuction opening 60. Pressure equalisation is necessary, however, inorder to reduce the excess pressure in the connecting tube 26 and thetube connections, 20, 22, and 24, and to apply the surrounding, oratmospheric, pressure to the reference side of the differential pressuresensors, 12, 14, and 16. Hence, an equalisation opening 30 is providedin the connecting tube 26, and is dimensioned such that, on the onehand, the predetermined pressure can be applied to the reference side ofthe differential pressure sensors, 12, 14, and 16, with the peristalticpump 18 and, on the other, the surrounding pressure can be applied as areference pressure to the reference side of the differential pressuresensors, 12, 14, and 16. After turning off the peristaltic pump 18, airescapes out of the connecting tube 26 through the equalisation opening30. Thus, pressure equalisation occurs through the equalisation opening30 between the surroundings and the reference side of the differentialpressure sensors, 12, 14, and 16, as shown by arrow 32.

In FIG. 2 a three-dimensional side view of one of the piezorestrictivedifferential pressure sensors, 12, 14, and 16, used in the apparatus isshown. The sensor 12 (14, 16) comprises a sensor housing 70, in which apiezorestrictive element is arranged. A connection support 74 isarranged on one side of the sensor housing 70 and comprises an opening78, through which a connection is established between the reference sideof the piezorestrictive element and the surrounding atmosphere. On theopposite side, the sensor housing 70 comprises a connection support 72,formed in the same way, which is connected to the measurement side ofthe piezorestrictive element. The sensor housing 70 further comprises arow of contact pins 76 that are electrically connected to thepiezoerestrictive element inside the sensor housing 70. A connection tothe control, or monitoring, unit 40 is established by means of thesecontact pins 76 through electrical leads, as shown in FIG. 1.

As is also shown in FIG. 1, the tube connection 20 (or the tubeconnection 24 or 26) is connected to the connection support 74, so thata predetermined pressure can be applied to the reference side of thepiezorestrictive element within the sensor housing 70 by means of theconnecting tube 26 and the peristaltic pump 18, as described in detail.

The connection support 72, which is connected to the measurement side ofthe piezorestrictive element, is connected to the blood carryingconduits, that are not shown here, in a known manner. In the embodimentdescribed above but not shown here, in which the sensors are arranged ina circuit, or a printed circuit, within the housing, the connectionsupport 72 is connected with terminals arranged on the housing wall bymeans of the connecting conduits, and the terminals are, in turn,connected to the blood-carrying conduits.

FIG. 3 shows a schematic view of the peristaltic pump 18. Theperistaltic pump 18 comprises a pump housing 80, on which is arranged amotor, of which only the motor axle 84 is shown. Moreover, there areprovided three freely rotating rolls 82, which are in direct contactwith the motor axle 84 and are driven by same. The connecting tube 26 islaid in the peristaltic pump 18, or the pump housing 80, in such a way,that it can be squashed between the rolls 82 and the wall of the pumphousing 80.

In order to generate excess pressure in the connecting tube 26, theperistaltic pump 18 is started under control of the control, ormonitoring, unit 40 (not shown). The motor axle 84 thus turns in thedirection indicated by arrow 88. This causes rotation of the rolls 82freely mounted in the pump housing 80 in the opposite direction, asindicated by arrow 86. As a result, the rolls 82 revolve within the pumphousing 80 in a counter-clockwise direction, as shown by the dashed linewith arrow 90 and, at the same time, squash the connecting tube 26against the wall of the pump housing 80, causing the suction of airthrough the suction opening 60, as shown by arrow 62.

The thus generated excess pressure in the connecting tube 26 is appliedby means of the tube connections, 20, 22, and 24, to the reference sidesof the differential pressure sensors, 12, 14, and 16, for testing thelatter, as described above in detail. After testing the differentialpressure sensors, the peristaltic pump 18 is turned off; the rolls 82then close the connecting tube 26 towards the suction opening 60. Anequalisation of pressure with the surroundings then occurs through theequalisation opening 30, so that the surrounding pressure can reach thereference sides of the piezorestrictive elements of the sensors, 12, 14and 16, as a reference pressure.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

What is claimed is:
 1. A method for testing a plurality of differentialpressure sensors, each of the plurality of differential pressure sensorsincluding a reference side and a measurement side, the methodcomprising: applying a first pressure having a first predetermined valueto the reference side of the plurality of differential pressure sensors,the first predetermined value being either a positive pressure value ora negative pressure value; measuring a first set of output values fromthe measurement side for each of the plurality of differential pressuresensors; comparing the first set of output values; applying a secondpressure having a second predetermined value to the reference sensorside of the plurality of differential pressure sensors, the secondpredetermined value being the negative pressure value if the firstpredetermined value is the positive pressure value or the secondpredetermined value being the positive pressure value if the firstpredetermined value is the negative pressure value; measuring a secondset of output values from the measurement side for each of the pluralityof differential pressure sensors; and comparing the second set of outputvalues.
 2. A method for testing a plurality of differential pressuresensors, each of the plurality of differential pressure sensorsincluding a reference side and a measurement side, the methodcomprising: applying a pressure having a predetermined value to thereference side of the plurality of differential pressure sensors;measuring output values from the measurement side for each of theplurality of differential pressure sensors; and comparing the outputvalues, wherein the differential pressure sensors are arranged in anapparatus for the extracorporeal treatment of blood.
 3. The method ofclaim 2, wherein the apparatus is a dialysis monitor.